The detection of short circuits in driver ICs (integrated circuits) for external driving transistors is mostly realized by monitoring the voltage of the respective transistors, e.g. the voltage between a drain and a source of a field effect transistor. However, the prevalent use of MOSFETs (metal-oxide field effect transistors) with ever lower on-state resistance introduces issues with respect to the protection scheme based on the measurement of the source drain voltage.
A diagram 100 shown in FIG. 1 illustrates a relation between a temperature of a substrate Tj being represented by an x-axis 102 of the diagram 100 and an on-state resistance RDS(on) of a typical MOSFET being represented by an y-axis 104 of the diagram 100, wherein the MOSFET may be provided on or embedded into the substrate. A first graph 106 shows the relation between the substrate temperature and the on-state resistance for a typical MOSFET, a second graph 108 shows the same relation for approximately 98% of MOSFETs which may be delivered by a manufacturer. In other words, the relation between the substrate temperature and the on-state resistance embodied by the second graph 108 represents an upper bound for 98% of MOSFETs, such that only 2% of MOSFETs from a delivery may have higher resistance values at the respective temperature values. The diagram clearly demonstrates a strong presence of a dependence of the on-state resistance RDS(on) of a MOSFET on the temperature Tj of the MOSFET. Within a typically rated range for the operating temperature of −40° C. to 180° C., for example, the on-state resistance may vary by up to a factor of 3.
In general, an operation state of a MOSFET where the source to drain voltage exceeds a certain threshold voltage is qualified as a short circuit state. In order to prevent a faulty qualification, that threshold voltage is usually set above values which may be reached by hot transistors conducting currents near or equivalent to the rated maximum on-currents. Such a threshold 110 is symbolically indicated by the dashed horizontal line in the diagram 100 shown in FIG. 1, wherein the continuous horizontal lines above and below the dashed horizontal line indicate exemplary standard deviations. A vertical arrow 112 indicates the “protection gap” which is present in the typical protection scheme relying on the monitoring of the source to drain voltage of a MOSFET, owing to the fact that, as explained, the on-state resistance is strongly dependent on the temperature of the MOSFET. In other words, the fixed threshold voltage 110 which is to be exceeded by the source to drain voltage of a MOSFET in the case of a short circuit is estimated assuming a hot MOSFET conducting a high current, e.g. a short circuit current. Therefore, there is a danger that a short circuit state during which a cold MOSFET with a low on-state resistance conducting an even higher current than the short circuit current might not be qualified as a short circuit. This is of course an undesired situation as it might quickly lead to fusing of the MOSFET and thereby to permanent failure of the device.
From theoretical calculations it can be further shown that the presumably quick process of a warming-up of a cold MOSFET conducting high currents might not take place fast enough to raise the on-state resistance of the MOSFET such that the source to drain voltage short circuit threshold voltage can be reached before permanent damage of the device. In general, the MOSFET will burn though before it can get sufficiently warm so that its on-state resistance RDS(on) can increase sufficiently enough for the source to drain voltage UDS=RDS(on)·I to reach or exceed the short circuit threshold voltage, whereupon protection mechanisms can be activated.
In order to close the “protection gap”112 indicated in diagram 100 in FIG. 1 in the described short circuit protection scheme, the temperature of the MOSFET during operation needs to be known. If the temperature of the MOSFET is roughly known, a cold MOSFET carrying a short circuit current can be distinguished from a hot MOSFET carrying a normal operating current, which would greatly improve the detection scheme.
So far, the temperature of MOSFETS is determined using temperature sensors which might be provided on the PCB (printed circuit board) on which the MOSFETS are arranged or using special temperature sensors which are arranged on MOSFETS. The PCB based temperature measurements have the disadvantage that the measured temperature only reflects a delayed and smoothed out temperature of the MOSFETS. Furthermore, an additional circuit needs to be provided for the evaluation of the signals provided by those sensors. The temperature sensors are mostly based on PTC (positive temperature coefficient) or NTC (negative temperature coefficient) elements. The second option allows for a precise determination of the temperature of a respective MOSFET to which the temperature sensor is attached or in which the temperature sensor is integrated. However, in that case expensive special MOSFETS and complex analyzing circuitry needs to be employed. Furthermore, both solutions also involve substantial additional expenditures.